Vertical profile of effective turbidity reconstructed from broadening of incoherent body-wave pulses-II.Application to Kamchatka data

Abstract

The vertical profile of effective turbidity under Kamchatka is reconstructed from observations of distance-dependent broadening of the inchoherent pulse of highfrequency body waves from small earthquakes, by means of a new approach and data processing scheme developed in Paper I. The key 'effective turbidity' parameter, g e , used is an immediate generalization of the common isotropic turbidity/scattering coefficient g. Measurements of 200-600 onset-to-peak delays for P and S waves for five Kamchatka stations are used for interpretation. The estimates based on these data correspond to the 2-4 Hz frequency band. The inversion of data is performed in terms of the parameters of two generic vertical effective turbidity structures: a piecewise-constant profile (PCP) and truncated-inverse-power-law profile (TPLP), both used in several variants. The variants of the inversions give consistent results, but also reveal rather limited resolution, not permitting the recovery of detailed profiles or a comparison of results among individual stations. The inversions indicate that the values of effective turbidity decay from the surface down: within the depth interval h=0-50 km, the decay is gradual; at greater depths it is much steeper, roughly following the inverse cube law. The estimates of average effective mean free path l e =1/g e are very close for P and S waves: 50-60 km (20 per cent) for the 0-20 km layer; 250-300 km (30 per cent) for the 20-80 km layer; and at h>60-80 km, l e #100(h/40)-2-4 for both P and S waves. The value of both the P-and the S-wave optical thickness (total scattering loss) of the upper 200 km is about 0.75 (25 per cent), and the lithosphericscattering contribution to t* P is estimated as 0.2 s at 1 Hz. The expected S-wave scattering loss agrees reasonably with the standard regional amplitude attenuation curve, probably reflecting the secondary role of intrinsic loss at 3 Hz. The S-wave scattering Q in the lithosphere of Kamchatka is estimated for f =1 Hz as 125, 205 and 255 for hypocentral distances of shallow events of 30, 100 and 300 km, respectively.